In the past, there have been a number of efforts to improve the performance of ballasts for gas discharge lamps. One direction such efforts have taken is to utilize electronic ballasts of the type having an input section for power factor and harmonic correction and an output section operating as a current-fed power resonant inverter. Active preregulator circuits have been used in an attempt to obtain high power factor and harmonic correction in such input sections. At the same time, the instant-start type of gas discharge lamps continue to be extremely popular, calling for ballasts which are compatible with instant-start lamps.
The use of active preregulators in instant-start applications has led to startup problems in that the integrated circuits used in such active preregulators take appreciable time to attain steady state operating conditions during start-up and can present undesirable operating conditions to the gas discharge lamps when passed through the inverter section during start-up transient conditions. For example, one integrated circuit useful in active preregulators typically takes 100 milliseconds up to 500 or 1,000 milliseconds to reach steady state operating conditions. At steady state conditions, the active preregulator provides 170 volts DC output, however, during transient start-up conditions the output is substantially below that. When operating instant-start lamps, this results in the undesirable effect of an unacceptably long xe2x80x9cpreheatxe2x80x9d or glow period at low voltage. For instant-start lamps, it is desirable to attain 90% of the steady-state operating voltage in less than 100 milliseconds, because longer preheat periods undesirably shorten lamp life due to excessive electrode erosion during such low-voltage preheat conditions. This is in addition to undesirable visible phenomena during starting. Solutions have been developed for the more expensive current-fed, power-resonant type of circuits, however, it is desirable to provide a simple, inexpensive solution for the less expensive voltage-fed type of circuit.
Additionally, the above-described problem of an unacceptably long preheat period is exacerbated by the use of voltage-fed circuits in place of current-fed circuits. It is, however, desirable to use voltage-fed circuits which are less expensive than current-fed circuits. The problem is exacerbated even more on multi-inverter systems where starting multiple lamps simultaneously may cause a dip in the supply bus voltage because the inverters draw a transient of current during lamp ignition, and, the cumulative effect of multiple transient currents temporarily overloads the voltage, source. It is, therefore, desirable to provide a simple, inexpensive solution to the harmful effects of multiple transient currents for the less expensive voltage-fed type of circuit.
In an exemplary embodiment of the present invention, a voltage-fed type of inverter for ballasting gas discharge lamps is provided. The inverter includes a d.c. bus, a reference bus, serially connected first and second inverter switches, each having a control terminal, between the d.c. bus and the reference bus, a control node for interconnecting the control terminals of the switches, a common node comprising the interconnection of the switches, a drive control circuit serially connected between the control node and the common node for regenerative control of the switches, a resonant inductor serially connected to a load circuit between the common node and the reference bus, and a delay circuit connected between the control node and the common node. The delay circuit delays the drive control circuit from starting regenerative control of the inverter switches for a predetermined period of time, allowing the d.c. bus to attain full steady-state operating voltage before the inverter starts.